The invention concerns an electrically rotatable adjusting shaft of a fully variable mechanical valve train of an internal combustion engine, said shaft comprising an adjusting cam.
The advantages of a throttle-free load regulation of Otto engines by means of fully variable inlet valve controls are known. By the omission of throttles, it is possible to exclude throttling losses that otherwise occur over a large range of load conditions of the internal combustion engine. This has a positive effect on fuel consumption and on the engine torque.
In variable mechanical valve trains, the stroke adjustment of the inlet gas exchange valves should be as spontaneous and exact as possible and should be effected at a high speed of adjustment. The adjusting mechanism is usually an adjusting shaft having locking curves or eccentrics.
Depending on the system used and the structural configuration, considerable moments of actuation are required for setting the desired valve stroke and the corresponding rotation of the adjusting shaft. These moments of actuation result from the reaction forces of the valve train that act on the adjusting shaft. For adjustment in a direction for obtaining a larger stroke, the adjusting shaft must be moved against the reaction forces of the valve train and, due to the oscillating movement of the gas exchange valves, this is accompanied by strongly pulsating torques.
To achieve an optimum operation of the valve train, a lash-free and extremely rigid support of the moments of the adjusting shaft is required. This support governs the positioning precision and the operation of a fully variable valve train as also the adjustability of an internal combustion engine equipped with such a system. The time for adjusting from a minimum to a maximum stroke should be less than 300 milliseconds.
The power requirement of the electric drive of the adjusting shaft should not put a too heavy load on the vehicle network. Therefore, small, high-speed electromotors combined with gearboxes having high transmission ratios are desirable.
One conceivable solution is to use worm drives. These, however, have a poor efficiency and are susceptible to wear that in its turn causes lash. In addition, worm drives have a limited range of transmission. It is also conceivable to use hydraulic adjusters similar to camshaft adjusters configured as vane-type adjusting devices or as coarse-thread adjusters. Their operation, however, depends to a large extent on the temperature of the lubricating oil. Besides this, they require an oil pressure that is only existent when the engine is running and their adjusting dynamics and rigidity are low.
A further solution may be rotary drives but these have a low efficiency and a great amount of rotational lash.
It is an object of the invention to provide a compact actuator for the adjusting shaft of a fully variable mechanical valve train of an internal combustion engine, which actuator should have the highest possible rigidity and possess characteristics of low lash and low friction.
This and other objects and advantages of the invention will become obvious from the following detailed description.
The invention achieves the above objects by the fact that an actuator for rotating the adjusting shaft comprises an adjusting lever that is connected rotationally fast to the adjusting shaft, and a free end of the adjusting lever is articulated on the screw nut of a screw-and-nut drive that is driven by an electromotor. The connection of the adjusting lever to the adjusting shaft as well as that of the adjusting lever and the screw nut to the threaded screw are substantially free of lash and very rigid. This results in a high positioning precision and, due to low frictional losses, a short adjusting time. Besides this, the actuator of the invention is very compact.
In a preferred embodiment of the invention, the electromotor drives the threaded screw but a solution in which a screw nut is driven is also feasible.
In an advantageous embodiment of the invention, the free end of the adjusting lever preferably comprises two fork branches that surround the screw nut with clearance. The fork shape of the adjusting lever permits the adjusting lever and the screw nut to be loaded symmetrically with a load that is free of bending moments. In contrast to one-sided loading by a simple adjusting lever, whose use is also conceivable, symmetric loading results in a higher rigidity. A further advantage is that the fork-shaped adjusting lever serves as an optimal securing device against rotation of the screw nut.
The arc-shaped movement of the free end of the adjusting lever and the linear movement of the screw nut necessitate a compensation of movement. This is achieved by the fact that the fork branches comprise opposing slots extending in longitudinal direction of the adjusting lever, and sliding blocks pivotally connected to the screw nut through bearing pins engage into the slots with clearance, said bearing pins having a common axis that extends through the center and the longitudinal axis of the screw nut.
Due to the fact that an electromotor shaft and the threaded screw are made together in one piece, no coupling is required between the electromotor and the threaded screw. Due to the relatively low lateral force exerted by the sliding blocks, no separate mounting arrangement is required for the threaded screw but only a fixed and a movable bearing for mounting the electromotor shaft. This simplifies the design of the actuator and gives it a compact structure while raising its rigidity.
The fixed bearing is configured as a deep groove or an angular contact ball bearing, or as a four-point bearing, while the movable bearing is configured as a needle roller bearing. Advantageously, the screw drive is preferably configured as a ball screw drive with pre-stress and ball deflection and is arranged on the side of the screw nut that is free of shearing forces. The low friction obtained with the balls permits the use of electromotors with a relatively low torque and despite high speeds of adjustment, the load on the vehicle network is only insignificant.
The low lash of the actuator resulting from the pre-stress is a basic requirement for a precise positioning of the adjusting shaft and, thus also, for an exact setting of the valve stroke.
The pre-stressing of the screw can be effected, for example, by an overdimensioning of the balls or, in multi-piece screw nuts, by pre-stressing the threaded parts of the nuts. In addition to a ball screw drive, a configuration as a roller screw drive is also possible.
The ball deflection arranged on the side of the screw nut that is free of shearing forces effects a trouble-free return of the balls.
Due to the fact that the transmission ratio between the electromotor and the adjusting shaft can be defined by the length of the adjusting lever and the pitch of the threaded screw, a transmission ratio between 50 and 500:1 can be realized in a single stage. The efficiency values that can thus be achieved are distinctly higher than with multi-stage rotary drives or with worm drives.
A further advantage of the invention is that the actuator can be installed in any longitudinal and any angular position on the adjusting shaft. In this way, the position of the actuator can be optimally adapted to the conditions of installation of the internal combustion engine.
Further features of the invention are disclosed in the following description and in the appended drawings which show a schematic representation of one example of embodiment of the invention.